The invention relates to digital camcorder video apparatus and, more particularly, to digital camcorder video apparatus using video compression techniques that are compatible with MPEG-2 decoding apparatus.
A concurrently filed patent application of the inventors entitled xe2x80x9cVIDEO TRANSMISSION APPARATUS EMPLOYING INTRA-FRAME-ONLY VIDEO COMPRESSION THAT IS MPEG-2 COMPATIBLExe2x80x9d is incorporated herein by reference for its showing of apparatus that can be used in conjunction with the digital camcorder video apparatus described in this specification and its accompanying drawing.
Digital video cassette (DVC) electromagnetic tape recording is currently done using standards developed in a High-Definition Digital Video-Cassette-Recorder Conference. Five standards were established in that conference, including a standard for standard density (SD) recording in which one frame of NTSC video is recorded in 1350 syncblocks. These 1350 syncblocks, 90 syncblocks of accompanying audio and 44 syncblocks of overhead are distributed amongst ten successive helically recorded tracks on the electromagnetic recording tape. Syncblocks are uniform in bit length, and five syncblocks comprise a segment of five macroblocks of DCT. Each block of DCT is based on an 8-pixel-by-8-pixel block of 4:2:0 image data. That is, luminance (Y) is sampled twice as densely in the horizontal direction and in the vertical direction as the red-minus-luminance color difference signal (Cr) and as the blue-minus-luminance color difference signal (Cb). Each macroblock contains four blocks of discrete cosine transform (DCT) descriptive of Y and two blocks descriptive of Cr and Cb, which blocks are of variable bit length. While there are 385 bytes per segment in the SD standard, often as few as a hundred or less are required for transmitting DCT capable of generating images of requisite resolution. One of the objectives of the inventors was to more efficiently utilize the bytes available in each successive group of ten tracks, so that the number of null bytes is reduced and so that image resolution is improved by using those previously unused bytes.
The HD Digital VCR Conference established a high-density (HD) baseband standard in which each frame of a high-resolution television picture, together with accompanying audio and overhead, occupies twenty consecutive recording tracks. The conference defined further standards for recording direct video broadcasting (DVB), advanced television (ATV), PAL+ for Europe and EDTV-II for Japan. For the most part, the recording scheme for DVB simply involves the payload being formed from segments of the transport stream for this transmission medium. A similar observation can be made concerning the recording scheme for DVB. However, there are rules for the insertion of data to support trickplay reproduction from the recording, in addition to normal reproduction.
The high-resolution TV picture contemplated for HD baseband recording is of MUSE type with 1125 scan lines and 1200 luminance pixels per scan line. Those skilled in the art recognize that the HD baseband standard does not conform to any of the formats supported by the high-definition broadcast television standard established by the Advanced Television Systems Committee. The ATSC standard supports 480 scan lines with 640 luminance pixels per interlaced scan line, 480 scan lines with 720 luminance pixels per interlaced or progressive scan line, 720 scan lines with 1280 luminance pixels per progressive scan line, and 1080 scan lines with 1920 luminance pixels per interlaced scan line. A known practice is to record two data segments of ATSC digital television signal, each preceded by a time stamp, in five syncblocks of DTV signal.
The audio signals used as source signals in TV broadcasting are sampled at a frequency of 48 kHz, locked to the 27 MHz system clock, and are encoded according to the digital audio compression (AC-3) standard specified in the body of ATSC document A/52. The resulting compressed audio information is parsed into packets identified in the packet headers as being audio packets.
The video signals used as source signals in TV broadcasting are encoded according to the MPEG-2 video compression standard. The resulting compressed video information is parsed into packets identified in the packet headers as being video packets. Transmission is by groups of pictures, each group of pictures (GOP) containing coding for an initial anchor frame referred to as an xe2x80x9cI framexe2x80x9d, subjected solely to intraframe video compression, followed by coding for a succession of other frames subjected to interframe compression coding. These other frames comprise so-called xe2x80x9cP framesxe2x80x9d and so-called xe2x80x9cB framesxe2x80x9d. Coding for each P frame is based on differences of that video frame in actuality from that frame as predicted by extrapolation from a most recent previous one of the I and P frames, in accordance with motion vectors derived by block comparison between the more recent of these previous I and P frames. Coding for each B frame is based on differences of that video frame in actuality from that frame as predicted by bidirectional interpolation from a preceding one and a succeeding one of the I and P frames.
MPEG-2 compressed video is suited to an application, such as television broadcasting, where there is not much concern with regard to the difficulty of editing video information in this transport stream format. In applications where ease of editing video is of concern, preferably video compression is done relying not at all on interframe compression techniques, but just on intraframe video compression techniques. Ease of editing video is desirable for editing of video recordings to delete undesired frames, to introduce frame repetition for achieving slow-motion or stop-motion effects, and to insert reverse motion sequences. By way of further examples, ease of editing video is also desirable for extracting still pictures from camcorder recordings, for extracting selected video for transmission over the internet and for editing commercials out of video recorded from broadcast television.
In systems for processing video in which ease of editing video is of concern, the inventors advocate intraframe video compression encoding every successive video frame in accordance with the way intraframe video compression encoding is done on I anchor frames in MPEG-2. Each frame is then identified as being intraframe video compression encoded in the picture headers, the same way it is done for anchor frames in MPEG-2. A conventional MPEG-2 encoder can be modified to carry out this video compression algorithm. Alternatively, the encoder for encoding just anchor or I frames can be considerably simplified from the encoder required for coding P frames and B frames as well as I frames, since there is no need for the motion estimation circuitry that forms a substantial part of a complete MPEG-2 encoder. The motion estimation circuitry requires memory with storage capability for plural frames of video information. The inventors prefer such a simplified encoder be used in a digital camcorder for reducing power drain on the camcorder battery and for shaving the weight and size of the camcorder.
In many systems for processing video in which ease of editing video is of concern, an MPEG-2 decoder is already available for use in decoding continuous intraframe video compression encoding descriptive of consecutive video frames. So there is no additional cost for a decoder to decode the transport stream of I frames without intervening P or B frames. If an MPEG-2 decoder is not already available in a system, providing such a decoder is reasonable in cost, since the amount of hardware in an MPEG-2 decoder is considerably less than that in an MPEG-2 encoder. Alternatively, a modified MPEG-2 decoder only for I frames can be used.
The invention is embodied in a camcorder comprising a video camera responsive to light images for supplying video signals on a frame by frame basis; circuitry responsive to the video signals supplied by the video camera for generating segments of a transport stream, which segments are susceptible to decoding by an MPEG-2 decoding apparatus; and a recorder for electromagnetically recording the transport stream as magnetic variations along the surface of a magnetic recording medium.